Refurbs IMO are a crapshoot. You may get a perfect one, you may get one with some scratches and/or dents on it. Keep in mind that if you do get one with scratches or dents you have pretty close to a 100% chance that Apple WILL NOT replace it as it doesn't effect the way the computer works. All refurbs come with the same 1 yr warranty as a new one would. Any defective parts are replaced before its sent to you orignally and then marked as a refurb. Sometimes its just a case where someone returned it for something else, but Apple can't sell it as new so it has to be sold as a refurb. Lets hope you get one of those if you get a refurb!

I myself have never had a refurb so I can't say from personal experience, but IMO, you do take a small chance when getting one. Most of the time you'll be ok with one.
As far as iMac or MBP? Well, do you need to be mobile? The higher end iMacs and MBP with C2D have basically the same major specs with the exception of RAM. The MBP comes with 2GB vs 1 GB on the iMac. So do you want screen size or portability?
RAM.... I go to either
www.memorytogo.com or
www.4allmemory.com for my Macs. RAM from there is gaurenteed to work in the Macs its for and both have lifetime warranty on the RAM. I've never had a bad piece from either one, ever! If you want to go the more expensive, but safe route
www.crucial.com is your best bet.
The link below shows exactly how to install RAM into a MBP. Its actually quite easy to do. Just take your time and be careful.
http://docs.info.apple.com/article.html?artnum=303491
If you go with the iMac, below is a link to add/remove RAM with it:
http://docs.info.apple.com/article.html?artnum=303084
Both the iMac and MBP take the same exact RAM. (DDR2 SODIMM)
Make sure its DDR2 and NOT DDR. They're 2 different technologies that aren't backwards compatible.